Merge pull request #673 from matsjfunke/fix/fastmcp-union-types

fix: support FastMCP union types in tool parameter forms

client/src/components/DynamicJsonForm.tsx

@@ -15,12 +15,22 @@ interface DynamicJsonFormProps {
   maxDepth?: number;
 }
 
+const isTypeSupported = (
+  type: JsonSchemaType["type"],
+  supportedTypes: string[],
+): boolean => {
+  if (Array.isArray(type)) {
+    return type.every((t) => supportedTypes.includes(t));
+  }
+  return typeof type === "string" && supportedTypes.includes(type);
+};
+
 const isSimpleObject = (schema: JsonSchemaType): boolean => {
   const supportedTypes = ["string", "number", "integer", "boolean", "null"];
-  if (schema.type && supportedTypes.includes(schema.type)) return true;
+  if (schema.type && isTypeSupported(schema.type, supportedTypes)) return true;
   if (schema.type === "object") {
     return Object.values(schema.properties ?? {}).every(
-      (prop) => prop.type && supportedTypes.includes(prop.type),
+      (prop) => prop.type && isTypeSupported(prop.type, supportedTypes),
     );
   }
   if (schema.type === "array") {
@@ -181,7 +191,13 @@ const DynamicJsonForm = ({
     const isRequired =
       parentSchema?.required?.includes(propertyName || "") ?? false;
 
-    switch (propSchema.type) {
+    let fieldType = propSchema.type;
+    if (Array.isArray(fieldType)) {
+      // Of the possible types, find the first non-null type to determine the control to render
+      fieldType = fieldType.find((t) => t !== "null") ?? fieldType[0];
+    }
+
+    switch (fieldType) {
       case "string": {
         if (
           propSchema.oneOf &&
@@ -337,6 +353,8 @@ const DynamicJsonForm = ({
             required={isRequired}
           />
         );
+      case "null":
+        return null;
       case "object":
         if (!propSchema.properties) {
           return (

client/src/components/ToolsTab.tsx

@@ -7,7 +7,11 @@ import { TabsContent } from "@/components/ui/tabs";
 import { Textarea } from "@/components/ui/textarea";
 import DynamicJsonForm from "./DynamicJsonForm";
 import type { JsonValue, JsonSchemaType } from "@/utils/jsonUtils";
-import { generateDefaultValue, isPropertyRequired } from "@/utils/schemaUtils";
+import {
+  generateDefaultValue,
+  isPropertyRequired,
+  normalizeUnionType,
+} from "@/utils/schemaUtils";
 import {
   CompatibilityCallToolResult,
   ListToolsResult,
@@ -104,7 +108,7 @@ const ToolsTab = ({
                 </p>
                 {Object.entries(selectedTool.inputSchema.properties ?? []).map(
                   ([key, value]) => {
-                    const prop = value as JsonSchemaType;
+                    const prop = normalizeUnionType(value as JsonSchemaType);
                     const inputSchema =
                       selectedTool.inputSchema as JsonSchemaType;
                     const required = isPropertyRequired(key, inputSchema);
@@ -148,7 +152,10 @@ const ToolsTab = ({
                             onChange={(e) =>
                               setParams({
                                 ...params,
-                                [key]: e.target.value,
+                                [key]:
+                                  e.target.value === ""
+                                    ? undefined
+                                    : e.target.value,
                               })
                             }
                             className="mt-1"

client/src/components/__tests__/DynamicJsonForm.test.tsx

@@ -35,6 +35,18 @@ describe("DynamicJsonForm String Fields", () => {
       const input = screen.getByRole("textbox");
       expect(input).toHaveProperty("type", "text");
     });
+
+    it("should handle a union type of string and null", () => {
+      const schema: JsonSchemaType = {
+        type: ["string", "null"],
+        description: "Test string or null field",
+      };
+      render(
+        <DynamicJsonForm schema={schema} value={null} onChange={jest.fn()} />,
+      );
+      const input = screen.getByRole("textbox");
+      expect(input).toHaveProperty("type", "text");
+    });
   });
 
   describe("Format Support", () => {

client/src/utils/__tests__/schemaUtils.test.ts

@@ -1,6 +1,7 @@
 import {
   generateDefaultValue,
   formatFieldLabel,
+  normalizeUnionType,
   cacheToolOutputSchemas,
   getToolOutputValidator,
   validateToolOutput,
@@ -142,6 +143,189 @@ describe("formatFieldLabel", () => {
   });
 });
 
+describe("normalizeUnionType", () => {
+  test("normalizes anyOf with string and null to string type", () => {
+    const schema: JsonSchemaType = {
+      anyOf: [{ type: "string" }, { type: "null" }],
+      description: "Optional string parameter",
+    };
+
+    const normalized = normalizeUnionType(schema);
+
+    expect(normalized.type).toBe("string");
+    expect(normalized.anyOf).toBeUndefined();
+    expect(normalized.description).toBe("Optional string parameter");
+  });
+
+  test("normalizes anyOf with boolean and null to boolean type", () => {
+    const schema: JsonSchemaType = {
+      anyOf: [{ type: "boolean" }, { type: "null" }],
+      description: "Optional boolean parameter",
+    };
+
+    const normalized = normalizeUnionType(schema);
+
+    expect(normalized.type).toBe("boolean");
+    expect(normalized.anyOf).toBeUndefined();
+    expect(normalized.description).toBe("Optional boolean parameter");
+  });
+
+  test("normalizes array type with string and null to string type", () => {
+    const schema: JsonSchemaType = {
+      type: ["string", "null"],
+      description: "Optional string parameter",
+    };
+
+    const normalized = normalizeUnionType(schema);
+
+    expect(normalized.type).toBe("string");
+    expect(normalized.description).toBe("Optional string parameter");
+  });
+
+  test("normalizes array type with boolean and null to boolean type", () => {
+    const schema: JsonSchemaType = {
+      type: ["boolean", "null"],
+      description: "Optional boolean parameter",
+    };
+
+    const normalized = normalizeUnionType(schema);
+
+    expect(normalized.type).toBe("boolean");
+    expect(normalized.description).toBe("Optional boolean parameter");
+  });
+
+  test("normalizes anyOf with number and null to number type", () => {
+    const schema: JsonSchemaType = {
+      anyOf: [{ type: "number" }, { type: "null" }],
+      description: "Optional number parameter",
+    };
+
+    const normalized = normalizeUnionType(schema);
+
+    expect(normalized.type).toBe("number");
+    expect(normalized.anyOf).toBeUndefined();
+    expect(normalized.description).toBe("Optional number parameter");
+  });
+
+  test("normalizes anyOf with integer and null to integer type", () => {
+    const schema: JsonSchemaType = {
+      anyOf: [{ type: "integer" }, { type: "null" }],
+      description: "Optional integer parameter",
+    };
+
+    const normalized = normalizeUnionType(schema);
+
+    expect(normalized.type).toBe("integer");
+    expect(normalized.anyOf).toBeUndefined();
+    expect(normalized.description).toBe("Optional integer parameter");
+  });
+
+  test("normalizes array type with number and null to number type", () => {
+    const schema: JsonSchemaType = {
+      type: ["number", "null"],
+      description: "Optional number parameter",
+    };
+
+    const normalized = normalizeUnionType(schema);
+
+    expect(normalized.type).toBe("number");
+    expect(normalized.description).toBe("Optional number parameter");
+  });
+
+  test("normalizes array type with integer and null to integer type", () => {
+    const schema: JsonSchemaType = {
+      type: ["integer", "null"],
+      description: "Optional integer parameter",
+    };
+
+    const normalized = normalizeUnionType(schema);
+
+    expect(normalized.type).toBe("integer");
+    expect(normalized.description).toBe("Optional integer parameter");
+  });
+
+  test("handles anyOf with reversed order (null first)", () => {
+    const schema: JsonSchemaType = {
+      anyOf: [{ type: "null" }, { type: "string" }],
+    };
+
+    const normalized = normalizeUnionType(schema);
+
+    expect(normalized.type).toBe("string");
+    expect(normalized.anyOf).toBeUndefined();
+  });
+
+  test("leaves non-union schemas unchanged", () => {
+    const schema: JsonSchemaType = {
+      type: "string",
+      description: "Regular string parameter",
+    };
+
+    const normalized = normalizeUnionType(schema);
+
+    expect(normalized).toEqual(schema);
+  });
+
+  test("leaves anyOf with non-matching types unchanged", () => {
+    const schema: JsonSchemaType = {
+      anyOf: [{ type: "string" }, { type: "number" }],
+    };
+
+    const normalized = normalizeUnionType(schema);
+
+    expect(normalized).toEqual(schema);
+  });
+
+  test("leaves anyOf with more than two types unchanged", () => {
+    const schema: JsonSchemaType = {
+      anyOf: [{ type: "string" }, { type: "number" }, { type: "null" }],
+    };
+
+    const normalized = normalizeUnionType(schema);
+
+    expect(normalized).toEqual(schema);
+  });
+
+  test("leaves array type with non-matching types unchanged", () => {
+    const schema: JsonSchemaType = {
+      type: ["string", "number"],
+    };
+
+    const normalized = normalizeUnionType(schema);
+
+    expect(normalized).toEqual(schema);
+  });
+
+  test("handles schemas without type or anyOf", () => {
+    const schema: JsonSchemaType = {
+      description: "Schema without type",
+    };
+
+    const normalized = normalizeUnionType(schema);
+
+    expect(normalized).toEqual(schema);
+  });
+
+  test("preserves other properties when normalizing", () => {
+    const schema: JsonSchemaType = {
+      anyOf: [{ type: "string" }, { type: "null" }],
+      description: "Optional string",
+      minLength: 1,
+      maxLength: 100,
+      pattern: "^[a-z]+$",
+    };
+
+    const normalized = normalizeUnionType(schema);
+
+    expect(normalized.type).toBe("string");
+    expect(normalized.anyOf).toBeUndefined();
+    expect(normalized.description).toBe("Optional string");
+    expect(normalized.minLength).toBe(1);
+    expect(normalized.maxLength).toBe(100);
+    expect(normalized.pattern).toBe("^[a-z]+$");
+  });
+});
+
 describe("Output Schema Validation", () => {
   const mockTools: Tool[] = [
     {

client/src/utils/jsonUtils.ts

@@ -21,7 +21,16 @@ export type JsonSchemaType = {
     | "boolean"
     | "array"
     | "object"
-    | "null";
+    | "null"
+    | (
+        | "string"
+        | "number"
+        | "integer"
+        | "boolean"
+        | "array"
+        | "object"
+        | "null"
+      )[];
   title?: string;
   description?: string;
   required?: string[];

client/src/utils/schemaUtils.ts

@@ -145,6 +145,95 @@ export function isPropertyRequired(
   return schema.required?.includes(propertyName) ?? false;
 }
 
+/**
+ * Normalizes union types (like string|null from FastMCP) to simple types for form rendering
+ * @param schema The JSON schema to normalize
+ * @returns A normalized schema or the original schema
+ */
+export function normalizeUnionType(schema: JsonSchemaType): JsonSchemaType {
+  // Handle anyOf with exactly string and null (FastMCP pattern)
+  if (
+    schema.anyOf &&
+    schema.anyOf.length === 2 &&
+    schema.anyOf.some((t) => (t as JsonSchemaType).type === "string") &&
+    schema.anyOf.some((t) => (t as JsonSchemaType).type === "null")
+  ) {
+    return { ...schema, type: "string", anyOf: undefined };
+  }
+
+  // Handle anyOf with exactly boolean and null (FastMCP pattern)
+  if (
+    schema.anyOf &&
+    schema.anyOf.length === 2 &&
+    schema.anyOf.some((t) => (t as JsonSchemaType).type === "boolean") &&
+    schema.anyOf.some((t) => (t as JsonSchemaType).type === "null")
+  ) {
+    return { ...schema, type: "boolean", anyOf: undefined };
+  }
+
+  // Handle anyOf with exactly number and null (FastMCP pattern)
+  if (
+    schema.anyOf &&
+    schema.anyOf.length === 2 &&
+    schema.anyOf.some((t) => (t as JsonSchemaType).type === "number") &&
+    schema.anyOf.some((t) => (t as JsonSchemaType).type === "null")
+  ) {
+    return { ...schema, type: "number", anyOf: undefined };
+  }
+
+  // Handle anyOf with exactly integer and null (FastMCP pattern)
+  if (
+    schema.anyOf &&
+    schema.anyOf.length === 2 &&
+    schema.anyOf.some((t) => (t as JsonSchemaType).type === "integer") &&
+    schema.anyOf.some((t) => (t as JsonSchemaType).type === "null")
+  ) {
+    return { ...schema, type: "integer", anyOf: undefined };
+  }
+
+  // Handle array type with exactly string and null
+  if (
+    Array.isArray(schema.type) &&
+    schema.type.length === 2 &&
+    schema.type.includes("string") &&
+    schema.type.includes("null")
+  ) {
+    return { ...schema, type: "string" };
+  }
+
+  // Handle array type with exactly boolean and null
+  if (
+    Array.isArray(schema.type) &&
+    schema.type.length === 2 &&
+    schema.type.includes("boolean") &&
+    schema.type.includes("null")
+  ) {
+    return { ...schema, type: "boolean" };
+  }
+
+  // Handle array type with exactly number and null
+  if (
+    Array.isArray(schema.type) &&
+    schema.type.length === 2 &&
+    schema.type.includes("number") &&
+    schema.type.includes("null")
+  ) {
+    return { ...schema, type: "number" };
+  }
+
+  // Handle array type with exactly integer and null
+  if (
+    Array.isArray(schema.type) &&
+    schema.type.length === 2 &&
+    schema.type.includes("integer") &&
+    schema.type.includes("null")
+  ) {
+    return { ...schema, type: "integer" };
+  }
+
+  return schema;
+}
+
 /**
  * Formats a field key into a human-readable label
  * @param key The field key to format